fuel pressure with 12psi of boost?

[black240jl]

what should the fuel pressure psi be set at with 12lbs of boost on an rb20det

[fast_s13]

one bar.... upping the fuel pressure isnt going to do anygood you need bigger injectors

[Bluefire]

fuel pressure should always be set at 42 psi with vac off... and around 35-36 with vac line on. You should be fine bumping the boost anyways, the fpr raises fuel pressure 1:1 with boost.

[black240jl]

oh i see, what is stock fp at idle??, and what do you mean vac on vac off??

[Darius]

There is a vacuum line on the fuel pressure regulator. To make sure your stock unit is working properly, it should be holding 43 psi with the vacuum line disconnected.

Since the fuel pressure rises 1:1 with boost, at 12 psi of boost the fuel pressure would be 43psi + 12psi=55psi.

Capishe?

[RPS13.5]

WHAT! I am confused???so how do you adjust the regulator? when the vacuum is off or on?is it34psi + 12psi boost = 46 psi fuel press with vacuumOR43psi + 12psi boost = 55 psi fuel press without vacuum

[mello88]

WHAT! I am confused???so how do you adjust the regulator? when the vacuum is off or on?is it34psi + 12psi boost = 46 psi fuel press with vacuumOR43psi + 12psi boost = 55 psi fuel press without vacuum

Only a NISMO FPR (or other aftermarket FPR) is adjustable. To adjust it , you idle the engine and disconnect hte vac hose from the FPR. There is a jamnut you loosen, then adjust the long screw of the FPR... Watch your fuel pressure gauge until it goes to 43.5psi (aka 3 bar). Then lock the jam nut down and reconnect the vac hose. With engine vacuum at idle the fuel pressure will drop somewhere around 37psi. That is the only time you will ever mess with the FPR....

Like mentioned before the fpr raises rail pressure at a 1:1 ratio with intake pressure...

The fuel pressure regulator is ALWAYS ran with the vac hose connected, unless you are setting it initially...

[RPS13.5]

With engine vacuum at idle the fuel pressure will drop somewhere around 37psi. That is the only time you will ever mess with the FPR....

Like mentioned before the fpr raises rail pressure at a 1:1 ratio with intake pressure...

I see that is easy to set up...Can I raised the pressure to 43 psi? and what is the safe pressure for 480cc injectors?

One thing I dint understand is 1:1 ratio - is that a actual or equal pressure scale?

[Carl H]

1:1 is 1:1 for every pound of boost fuel pressure increases a pound.

[mello88]

Can I raised the pressure to 43 psi? and what is the safe pressure for 480cc injectors?One thing I dint understand is 1:1 ratio - is that a actual or equal pressure scale?

No don't mess with your fuel pressure. Set it to 43.5psi without the vac hose, then hook up the vac hose and let it fall wherever it wants to (most likely around 37psi). You're done, put the tools away and go driving.

1:1 means for every 1 psi of boost in your intake manifold the fuel pressure is raised by 1psi. Otherwise the pressure across the fuel injector would change and you would not flow the same amount of fuel under boost.... The same is true under vacuum, the fuel pressure must be lowered at 1:1 ratio or else you would flow too much fuel at idle.

hope that helps...

[RPS13.5]

I see how that works... So its still better to get 555cc vs. 480cc+FPR....

Thank you all for the info, and that is a lot of great help here in NICO.

[mello88]

Yes, get the larger injectors rather than trying to mess with the rail pressure to compensate for too-small injectors. What is your power goal? The 550s should be good for ~450bhp at 80% duty cycle

here is a link with more info on injector mathhttp://www.rceng.com/technical.aspx

PS fuel pressure regulator usually isn't an optional thing if you've stuck a [walbro] pump in the tank.. The stock FPR usually can't control the rail pressure effectively so most people tend to put on a Nismo or go with a Sard, AEM, Aeromotive, etc FPR.. It's not really to raise the rail pressure with, it's to set it to 43.5 and leave it alone.

[Darius]

I still run the stock FPR and it still showed 43.5psi off vacuum when I installed the Denso Supra TT pump. The pump has nothing to do with it unless it is flowing too much gas that it can't pass the FPR during idle.

The purpose of an adjustable FPR is to raise the fuel pressure to compensate for too small of injectors. This assumes the pump that you have installed can pump more gas at that elevated pressure. Higher fuel pressure means more gas for the same injector cycle duration.

[RPS13.5]

I have a walbro in the tank already.So far the motor run 225whp/205Torque, I am not sure how much is at the crank? any conversion or calculator that I can see???My goal is 300hp at the crank, so I am not sure how much is at wheel HP? I guess 555cc will be great for 300hp at crank, and it may be little to high for 480cc

At RC calculatorfor 480cc can achieve 75hp each injectors & 300hp for 4 injectorsI am not sure if HP at the wheels or at the crank???

It seems that 480cc can do 300hp max, is that bad for the injectors?
Modified by RPS13.5 at 1:05 PM 6/5/2008

[mello88]

ok.. A general rule of thumb for injector sizing is figure out 68% of the flow rate. Thats will tell you the aprox max hp at the wheels, for six of those injectors at ~80% duty cycle, figuring in a 15% driveline loss...

With a 6 cyl motor, if you take one injector and look at it's flow rate (say 480cc) that # is also a rough estimate of the max crank horsepower all 6 can do at 100% duty cycle (so ~480hp). You wanna stay under 80% duty cycle, so that drops it to (480 * .80 =) ~380hp at the crank. On M/T car you lose 15% to 20% through the driveline, that ends up (~380 *.80 =) ~300-325 at the wheels (or 68% of the flow rate of 1 injector...)

Of course you can squeeze more out of them by raising the fuel pressure, running at a higher duty cycle, or changing AFR... But at that point you probably want to run a larger size anyway.

If you currently have 225whp.. Add 15% to compensate for driveline loss and you're at ~260bhp (at the crank). Stock RB25 injectors are 370cc, use the math above (370*.68 = 251whp) and you see you're getting close to the safe limits for your injectors... I say go with the 550's so you have room to play, if all you want is ~300-375 at the wheels.

[RPS13.5]

You know what I'll just get 555cc never mess anything else

thanks for the hint...

[tunermt]

There is a vacuum line on the fuel pressure regulator. To make sure your stock unit is working properly, it should be holding 43 psi with the vacuum line disconnected.

Since the fuel pressure rises 1:1 with boost, at 12 psi of boost the fuel pressure would be 43psi + 12psi=55psi.

Capishe?

For a standard RB this is correct 43psi fuel
Pressure with vacuum reference removed. You should always run a 1:1 fuel pressure regulator (standard) and being we now live in the future and technology has advanced so much anyone running a standalone ecu should absolutely run a fuel pressure sensor to the ecu. A $100 sensor can save an engine if there is a fuel system failure or problem when the calibration is all setup correctly.
https://youtu.be/BH3Rg37sjHk?si=WbA0eggg_gogtktP

[tunermt]

Side note on my rb25 I run 65psi base pressure to squeeze out a little extra injector flow so I can run e85 (also using flex fuel sensor of course).